If ammonia and carbon dioxide are reacted in a urea synthesis zone at elevated pressure and temperature suitable for the formation of urea, the reaction proceeds first to the formation of ammonium carbamate, and then, with a split-off of water, this ammonium carbamate is partly converted into urea. The aqueous urea solution leaving the urea synthesis zone thus contains nonconverted ammonium carbamate. Furthermore, if an excess of ammonia is introduced into the urea synthesis reactor, as is normally done, this aqueous urea solution will also contain free unreacted ammonia.
This urea and ammonium carbamate containing process stream is fed from the urea synthesis zone to one or more ammonium carbamate decomposition zones wherein, through a combination of process steps which may include expansion, heating and/or stripping, the ammonium carbamate is decomposed and the resulting ammonia and carbon dioxide, along with any free ammonia initially present in the process stream, is removed leaving an aqueous solution of urea. In most commercial processes, the gas mixture of ammonia, carbon dioxide and water vapor separated from the aqueous urea solution is absorbed and/or condensed and returned to the urea synthesis zone as an aqueous solution of ammonimum carbamate. However, the drawback of this procedure is that water is introduced into the urea synthesis reactor where it impedes the conversion of ammonium carbamate into urea. This, in turn, results in a higher concentration of ammonium carbamate leaving the urea synthesis zone and therefore a higher energy consumption to decompose the ammonium carbamate and separate it from the aqueous urea solution. Earlier processes have been successful, through a combination of expansion and heating steps, in minimizing the quantity of water recycled with the aqueous ammonium carbamate solution. However, in such an aqueous solution recycle process some water must necessarily be present to maintain the ammonium carbamate in solution.
Other processes have proposed recycling the ammonium carbamate as a solid slurry of ammonium carbamate in an inert oil, or as a hot gas mixture of ammonia and carbon dioxide. In the oil slurry processes, additional equipment is required to form the slurry and to separate the oil from the finished product. In the hot gas process, enormous quantities of energy must be expended to compress the gas mixture, which must be maintained at an elevated temperature in order to prevent the deposition of solid ammonium carbamate. Neither of these proposals have gained significant commercial acceptance.
It has also been proposed to separately compress and recycle the ammonia and/or carbon dioxide to avoid the formation of solid ammonium carbamate. However, mixtures of ammonia and carbon dioxide form a maximum boiling azeotrope and therefore resist separation by conventional means. Therefore, it has been proposed, for example in German Pat. Spec. No. 669,314, to separate the ammonia and carbon dioxide from the gas mixture resulting from the decomposition of ammonium carbamate by selective absorption of carbon dioxide in, for example, a monoethanolamine solution. However, this has the disadvantage that the absorbed carbon dioxide must thereafter be removed from the absorption agent by heating.
It has further been proposed to absorb the gaseous mixture of ammonia and carbon dioxide in water or an aqueous solution, thereafter remove free ammonia from the solution by distillation at atmospheric pressure and subsequently separately remove carbon dioxide by fractional distillation at a higher pressure of from between 5 to 20 atmospheres. A process of this type is disclosed in British Pat. Specification No. 1,129,939. However, the drawback of processes of this type is that it is first necessary to expand the gas mixture to atmospheric pressure and thereafter bring the remaining solution up to a pressure of 5 to 20 atmospheres after the ammonia has been removed. Moreover, the ammonia is obtained at atmospheric pressure and if it is to be returned to the urea synthesis zone, energy must be expended to compress it up to the urea synthesis pressure.